Hydrophobically-modified urethane thickeners are known in the art. See, for example, U.S. Pat. Nos. 4,079,028 and 4,155,892. These thickeners are condensation polymers of polyether polyols and polyisocyanates. The ends of the polymer chains may be "capped" or reacted with a hydrophobic material to introduce hydrophobic groups onto the ends of the polymer chains.
The hydrophobic groups may be introduced into the polymers in a number of ways. For example, if the condensation polymers are made with an excess of polyisocyanates, the resultant polymer has isocyanate end groups which may be reacted with at least one monofunctional hydrophobic organic compound. The monofunctional hydrophobic organic compound may be a hydrophobic alcohol or amine. If the condensation polymer is terminated with a hydroxide, then the polymers may be reacted with a long carbon chain monoisocyanate.
A number of variations to these condensation polymers have been made, including, for example, the substitution of the urethane linkage with other functional groups. See, for example, U.S. Pat. Nos. 4,079,028; 4,155,892; 4,298,511; 4,327,008; 4,337,184; 4,373,083; 3,770,684; 4,426,485; 4,411,819; 2,948,691; 4,209,333 and 4,304,902.
The hydrophobic groups of these thickening polymers are capable of non-specific associations, such as adsorption on surfaces including latex particles and self-aggregation in solution. These associations yield a network of the thickener molecule backbones. It is important to determine the mechanism by which these so-called "associative" thickeners work in order to understand and control the thickening caused them.